To separate the synthesis gas components, industrial installations use in particular the condensation process and the methane scrub; these are both cryogenic processes. A precondition for these processes to be used is for the gas fed to the cryogenic gas separation unit to be free of water, methanol, carbon dioxide (CO2) and other compounds which at the low temperatures used would lead to the formation of solids and therefore to the heat exchangers becoming clogged. Therefore, the crude synthesis gas is subjected to a pre-treatment comprising a plurality of steps, during which it for example passes through a CO conversion and is purified in a CO2 scrub removing the majority of the undesirable substances. The remaining residues are removed by adsorption in a subsequent adsorber station.
The adsorber stations used comprise at least two adsorbers which can be regenerated and each contain at least one adsorber bed (fixed bed of one or more suitable adsorption agents). The pre-purified crude synthesis gas flows through the adsorber bed and as it does so releases the undesirable substances which it contains (e.g. CO2 is still present in amounts of <50 mol ppm) to the adsorption agent(s). After the adsorption time has elapsed, a switch takes place from the adsorber laden with undesirable substances to an unladen adsorber of the adsorber station. The laden adsorber is then regenerated, i.e. is purified so as to remove the substances that it has previously adsorbed.
The capacity of the adsorption agents used is highly temperature-dependent. To regenerate an adsorber, therefore, it is first of all isolated from the synthesis gas stream and then hot regeneration gas flows through it in the opposite direction to the direction of flow of the crude synthesis gas. The substances which are desorbed from the adsorber bed at the elevated temperature are discharged from the adsorber together with the regeneration gas. The regeneration gas laden with the desorbed substances is treated differently depending on its composition.
The regeneration gas used is generally part of the H2 fraction (H2 content>approx. 90 mol %, pressurized) from the cryogenic gas separation unit. If high-purity hydrogen is to be generated as a product in the installation in addition to CO, the laden regeneration gas is cooled, condensate which forms is separated off, the laden regeneration gas is mixed with the remaining H2 fraction and then fed to a pressure swing adsorption (PSA) installation, where high-purity hydrogen is obtained and released as product at the end of the installation, while the remaining substances, which are produced in the residual gas at low pressure (including any CO which may be present) are then fed for undergrate firing. After the regeneration, the adsorber is available again for purifying the crude synthesis gas.
Although the selectivity of the adsorption agents used is high, in addition to the undesirable substances CO— albeit to a lesser extent—is also co-adsorbed and removed from the synthesis gas. If the partial pressure of the carbon monoxide in the regeneration gas is lower than in the crude synthesis gas, some of the adsorbed CO is desorbed during the regeneration. Since in the next adsorption phase (purification of the crude synthesis gas) CO is adsorbed again from the crude synthesis gas, an adsorber regeneration carried out in this way can lead to fluctuations in the composition of the synthesis gas entering the cryogenic gas separation unit and consequently also in the product quantities released at the end of the installation. Moreover, a fluctuating synthesis gas composition can lead to non-steady-state operation of the cryogenic gas separation unit, which under certain circumstances also leads to a reduction in product purity. Since the H2 fraction from the cryogenic gas separation unit which is normally used as regeneration gas has a lower CO content than the synthesis gas, this procedure gives rise to the negative phenomena which have been described above.
At the moment at which the crude synthesis gas stream is switched over to a freshly regenerated adsorber, the latter is still full of regeneration gas, which is displaced by the crude synthesis gas and transported into the cryogenic gas separation unit. If the regeneration gas has a different composition from the crude synthesis gas (for example in the case of regeneration using nitrogen (N2)), undesirable fluctuations in product quality can occur at least for a short time.
The residual gas fraction (mixture of various gases) produced in the cryogenic gas separation unit is generally not sufficient in quantitative terms to be sufficient to regenerate the adsorbers on its own. One technical solution involves increasing the residual gas quantity by admixing some of the H2 fraction. Since the residual gas fraction is at only a low pressure, the admixed H2 fraction as product is lost, and only its calorific value can be exploited. On account of the low pressure of the residual gas, it is necessary, before the synthesis gas is switched over to the freshly regenerated adsorber, for the latter firstly to be pressurized again using synthesis gas, which once again means fluctuations in product quantities and additional time. A corresponding reduction in pressure has to be carried out for the laden adsorber before the regeneration begins.
If the H2 fraction from the cryogenic gas separation unit can be released directly as product hydrogen, as is the case for example for the H2 fraction from the methane scrub, which comprises approx. 99 mol % hydrogen with a CO content of less than 10 mol ppm, this fraction likewise cannot be used for regeneration, since it is not permissible for the H2 product to be contaminated with CO. Furthermore, the use of the H2 fraction as regeneration gas gives rise to additional pressure loss so that an additional compressor may have to be used for the entire H2 quantity in order to reach the desired H2 product release pressure.
The use of product H2 or nitrogen which has to be supplied from outside the installation as regeneration gas is possible but constitutes a considerable cost factor. Furthermore, the use of N2 as regeneration gas can lead to an undesirably high nitrogen content in the Co product, at least for a short time. To prevent this, the freshly regenerated adsorber can be purged with synthesis gas before the crude gas is switched to this adsorber, but this again leads to losses and to a drop in product quantities.
While a synthesis gas installation is being started up, there is no gas or only an insufficient amount of gas available from the cryogenic gas separation unit for use as regeneration gas. In this case too, it is possible to use external N2 or product H2 as regeneration gases, but once again with the economic drawbacks which have been described above.